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Multiply the width X height of your slot to find square inch opening (3 X 10 = 30 sq in.). You want to convert this rectangular opening to the equivalent round opening.

Use the fomula pi (R squared). pi = 3.14 R is radius of a circle, half of the diameter. This formula gives you square inches of a circle. You want to find out what size circle will yield 30 sq in. in order to use the site's calculator.

30 = 3.14 X (radius X radius)

(By the way, the reason the formula was developed using radius instead of diameter is because 'radius' signifies a circle, whereas 'diameter' doesn't in all cases. A rational thought that just occurred to me.)

I tried to figure out how to go backwards through this calculation but I just settled instead for the good ol' trial and error that I love so much. So....after a couple of tries...with a radius of 3.1 in, you can plug that into the formula and find that the result is very close to 30 square inches. A radius of 3.1 indicates a diameter of 6.2. Trying it out a few times in the formula you find that the equivalent round port diameter would be between 6 1/8 and 6 1/4, or more precisely 6.1875 (a.k.a. 6 3/16). (Excuse me, but I'm in the habit of finding the closest real-life yardstick measurement when confronted with decimals :).

Input your box's dimensions in the site's rectangle box volume calculator. Use the box volume that you found and this port diameter, along with the port length, to find tuning frequency using the site's calculator. The port opening shape was changed but the port length remains exactly as you stated.

...and for the benefit of anyone who is reading this after doing a search, here is a port size calculator.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

port calculator